Method for manufacturing a composite sorber for the removal of h20 consisting of hygroscopic inorganic salts dissolved in a polymeric matrix

ABSTRACT

The present invention relates to a method for manufacturing H 2 O composite sorbers consisting of a polymeric matrix in which hygroscopic inorganic salts are dissolved, to composite sorbers consisting of hygroscopic inorganic salts dissolved in a polymeric matrix and their use for the removal of H 2 O from the housings of devices sensitive to the presence of H 2 O.

The presence of H₂O, even at trace levels, results to be harmful for thecorrect functioning of various devices, among which lithium batteries,microelectromechanical devices known in the field by the acronym MEMS(MicroElectroMechanichal Systems), organic displays of the OLED-type(Organic Light Emitting Diode) and photovoltaic cells, i.e. the cells ofthe OSC-type (Organic Solar Cells), to mention some of the mostinteresting devices. In the following these devices, and more in generalany sealed device in which the presence of H₂O results to be harmful,even at low levels (lower than 5000 ppm), are referred to by the term ofsensitive device.

In the sensitive devices the presence of H₂O can cause the progressivedeterioration of the performances; as an example, further informationwith respect to the effects of this contaminant can be found in thearticle “Correlation between dark spot growth and pinhole size inorganic light-emitting diodes”, by Shuang Fang Lim, et al., published inApplied Physics Letters, volume 78 no. 15, on 9 Apr. 2001 as relating tothe OLED displays, and in the fifth chapter of the book “OrganicPhotovoltaics—Concepts and Realization” by Brabec et al., edition of2003 by Springer-Verlag, as relating to the photovoltaic cells of theOSC-type.

The use of sorbers for the removal of gaseous impurities from thehousings of devices sensitive to their presence is known in the art. Forexample the international patent application WO 2004/072604 in the nameof the applicant discloses the use of active components dispersed insuitable porous matrices; the patent applications WO 2007/013118 and WO2007/013119, both in the name of the applicant, instead describenanostructured systems in which the active component is confined in aporous medium, itself being dispersed in a polymeric matrix, whereas theuse of functionalized nuclei dispersed in a permeable polymeric matrixis described in the international patent application WO 2007/074494 inthe name of applicant.

All of the foregoing technological solutions, although being effectivewith respect to the problem of H₂O removal, rely on complex techniquesand methodologies for carrying out them.

Other simpler technological solutions rely on a dispersion of activeelements in polymers, in which the dimensions of the particles areutilized to obtain secondary properties, as the transparency, of thesorber produced in this way.

This type of solution is described in the patent U.S. Pat. No.6,740,145, where it is requested that the dimensions of the particles ofthe dispersed active element are between 1 and 100 nm, and in the patentapplication US 2006/0097633 where a average particle size in thepolymeric film of less then 100 nm with a specific size distribution isrequested.

The patent application MI2007/A000690, in the name of the applicant andnot yet published on the filing date of present application, shows theuse of nanostructured fibres having inside an active component in formof dispersed particles.

The solutions shown in these documents have mainly two disadvantages,one inherent to the production and the other to the characteristics ofthe product. As relating to the disadvantage inherent to the production,this is associated to the use of particles of the active medium with awell defined particle size that requires also special precautions fortheir handling in case in which said particles have a very smalldiameter, i.e. 100 nm or less.

From the point of view of the efficiency of secondary characteristics ofthe product, i.e. not directly correlated to its H₂O sorbing capacity,as for example the transparency, the use of an active element in form ofparticles dispersed in a polymeric matrix can cause the deterioration inthe course of time, mainly due to problems of particle aggregationinside the dispersion medium. This problem is increasingly relevant, thelarger the particles sizes of the active medium are; the importance andcriticality of the particle size of the hygroscopic material are pointedout several times in the description of the above-mentioned patent U.S.Pat. No. 6,740,145. Object of the present invention is thus to overcomethe inconveniences still present in the prior art as relating to themanufacturing of a H₂O sorber containing a hygroscopic inorganic salt,with special reference to the necessity to use inorganic salts ofnanometric dimensions in which the particle size is of fundamentalimportance for the characteristics and properties of the producedsorber. In a first aspect thereof, the invention consists in a methodfor the manufacturing of a sorber for the removal of H₂O comprising apolymeric matrix and a hygroscopic inorganic salt, characterised in thatsaid hygroscopic inorganic salt is dissolved inside of said polymericmatrix. In the following the term composite material will be used toidentify the sorber material in order to highlight that is obtaineddissolving an inorganic compound in a polymerized organic mixture, evenif it is actually an homogeneous material that is not characterized byphase separation.

The invention will be illustrated in the following with reference to thedrawings, in which:

FIG. 1 shows a photography taken by an optical microscope of a compositesorber made according to the present invention;

FIG. 2 shows a photography taken by an optical microscope of a compositesorber made with one of the inorganic salts suitable for carrying outthe present invention, but not with the method herein described;

FIG. 3 shows a photography taken by an optical microscope of a compositesorber made with a non-suitable material;

FIG. 4 show the comparison of the transparency of different compositepolymeric films; and

FIG. 4A shows the comparison between the transparency of a film, madeaccording to the method of the invention, at time zero and of the samefilm after 23 hours of exposure to air.

Differently from the solutions of the prior art, for the manufacturingof the invention it is also possible to use powders with a particle sizelarger than 5 μm, and even, as will be illustrated in the following,also with a particle size larger than 2 mm; this simplifies theirstorage and handling, other than providing advantages in themanufacturing process, as will be illustrated further ahead.

The inventors have discovered various methods to manufacture the H₂Ocomposite sorbers consisting of a polymeric matrix and a dissolvedhygroscopic inorganic salt; in particular the polymer and the inorganicsalt are both dissolved in a solvent, the evaporation of which resultsin the consolidation of the polymeric matrix that thus contains thedissolved inorganic salt. In a preferred embodiment the polymer and theinorganic salt are dissolved in the same common solvent, but it is alsopossible to use two different solvents for the polymer and for thehygroscopic inorganic salt, in which case it is necessary that the twosolvents are miscible with each other.

In alternative it is possible to directly dissolve the hygroscopicinorganic salt in the polymer precursor, ideally when this is still inthe monomer state. Subsequently the polymerization process, for exampleby thermal treatment or UV irradiation, leads to the formation of thepolymeric matrix inside which the hygroscopic inorganic salt isdissolved.

The information on the solubility of polymers and inorganic salts insolvents, the miscibility of different solvents, the solubility ofinorganic salts in polymer precursors (their monomers, that in this caseact as solvent for the inorganic salt) are widely known and diffused andavailable to experts in the field and can be found, for example, inpublications as the Handbook of Chemistry and Physics, 87^(th) edition,2006-2007, edited by CRC or Alkaline Earth Metal Perchlorates, IUPACSolubility data Series, Vol. 41, edited by Pergamon Press. Solubilitydata can be moreover found in other publications, as for example theHandbook of Solubility Parameters and other Cohesion Parameters, 2^(nd)edition, 1991, or Hansen Solubility Parameters—A user's Handbook byCharles M. Hansen, 2^(nd) edition, 2007 both edited by CRC Press.

Whenever the solubility of a salt in a solvent, a monomer, a polymer ora mixture of at least two of them is not directly available inbibliographic data, an expert in the field can easily obtain it asexperimental value. In fact a simple solubility test can consist insubsequent addition of small amount of solute (salt) in a fixed amountof the liquid mixture: starting of precipitate formation correspond tothe solubility limit of the salt in the chosen mixture.

Hygroscopic inorganic salts suitable for carrying out the invention arealkaline-metal and alkaline earth-metal perhalogenates, alkaline-metaland alkaline earth-metal halides; the use of perchlorates is preferred.

In the case of composite sorbers that foresee the use of solvents, someexamples of polymers and solvents suitable for carrying out theinvention are cellulose acetate in methyl acetate, or the same celluloseacetate in tetrahydrofuran cellulose.

In the case in which the solution is composed only of the polymerprecursor and of the hygroscopic inorganic salt (the precursor acts assolvent for the inorganic salt) some examples of suitable polymers arepolymethylmethacrylate (PMMA), obtained by polymerization ofmethylmethacrylate (MMA), or polyethylmethacrylate (PEMA) obtained bypolymerization of ethyl methacrylate (EMA).

It is also possible to use combinations of polymers or polymerprecursors for obtaining the polymeric matrix of the composite sorberbeing object of the present invention. The case in which one of thesepolymers or polymeric precursors acts inside the solution ascross-linking agent results to be particularly advantageous. As anexample, the use of ethylene glycol dimethylacrylate as cross-linkingagent for obtaining a composite sorber having as polymeric matrix PMMAand as hygroscopic inorganic salt dissolved therein magnesiumperchlorate results to be particularly advantageous. Moreoverparticularly advantageous is the combination of a polymeric precursorwith a low molecular weight polymer. As an example, the use of mixtureof polymethylmetacrylate (PMMA) with methylmethacrylate (MMA) results tobe advantageous whenever the outgassing and shrinkage amount must to beminimized, i.e. if the polymerization has to take place in a closedenvironment or in a closed device.

Functionalized polymer precursors can moreover be added in order toimprove and optimize physiochemical material properties. The adhesion ona selected substrate in fact is very important for the final applicationof a composite sorber and the use of trimethylsiloxyethyl methacrylateresults to be very advantageous to improve the adhesion on a glass-typeor a metal-oxidized substrate.

Other additives are usually foreseen in the final composition in orderto act as polymerization promoter. Different kinds of polymerizationcatalysts can be used, as for example cationic, anionic or radicalinitiator, and it is chosen in function of the polymerization processthat will be used to obtain the final matrix. Its concentration isusually lower or equal to 1% w/w.

The quantity of inorganic salt to be added during the manufacturingprocess of the composite sorber depends on the specific hygroscopicinorganic salt used and on the characteristics of the polymer precursor(in the case in which this acts as solvent for the inorganic salt) or onthe solvent-polymer-inorganic salt combinations and is difficult todetermine a priori; and it is thus necessary to gradually add quantitiesof the inorganic salt to dissolve to the solution that will allow toobtain the polymeric composite sorber, proceeding with its dissolution,for example by stirring the solution and observing when the addition ofthe inorganic salt corresponds to the formation of a precipitate on thebottom of the solution. When this situation is reached this indicatesthat the addition of further inorganic salt quantities will not increasein any way the concentration of the dissolved inorganic salt, whereby itis possible to proceed to the consolidation of the polymeric matrix,upon eliminating the precipitated inorganic salt, for example by meansof filtration.

The use of hygroscopic inorganic salt powders with a particle sizelarger than 5 μm makes more evident the determination of the limitconditions being reached and simplifies the subsequent operations ofremoving the excess precipitate.

Obviously it is also possible that the saturation condition of thesolution is not reached, in which case the removal operation of theprecipitated inorganic salt is not necessary.

In a second aspect thereof the invention consists in a method for theremoval of H₂O from devices sensitive to its presence by the use ofcomposite sorbers consisting of hygroscopic inorganic salts dissolved ina polymeric matrix.

Among the sensitive devices that benefit the most from the applicationof the method of the invention are photovoltaic cells, OLED displays,microelectromechanical devices and lithium batteries.

In general the method of the invention offers advantages when it isnecessary that the H₂O concentration inside the sensitive device mustnot exceed a critical value during the normal functioning of the device.This critical value is related to the kind of sensitive device and amongthose which require a very low water concentration are the OLEDs, thattypically need concentrations in the order of 10 ppm or less, whereas atthe extreme opposite there are the solar cells, which can support up to5000 ppm before irreversible deterioration phenomena are triggered.

The polymer containing the dissolved hygroscopic inorganic salt may beused in form of an already consolidated film, in this case withthicknesses typically between 10 μm and 200 μm, or it may be used whennot yet completely solidified, while carrying out the final phase ofconsolidation or polymerisation once it has been dispensed: in this casethe use of the monomer as medium in which the hygroscopic inorganic saltis dissolved results to be particularly advantageous for this way ofuse, because there is no evaporation of the solvent that could give riseto contaminations of the device. The dispensing on the final support maybe carried out by various methods widely known in the field, for examplethrough brush work or spraying; preferred is the use of serigraphicmethod, well known in the printing field, which allows a better controlof the deposit thickness (by means of control of the stencil thickness,through which the mixture is forced to pass for reaching the support) oralso a filmograph may be used (a plate held at a fixed distance from abase or support, at a distance that corresponds to the thickness of thefilm).

Another way of using the sorbers made according to the present inventionforesees the melting of the polymer containing the dissolved hygroscopicinorganic salt; in this case it is useful to obtain the H₂O sorber froma thermoplastic polymer, i.e. from a polymeric material with a meltingtemperature typically lower than 300° C.

Another embodiment foresees the use of H₂O sorbers made according to thepresent invention in the form of nanofibres, that can be produced by atechnique known in the field by the term “electrospinning”.

In some devices it may be useful to fill the inner volume of thesensitive device with the composite sorber; in this case the preferredsolution foresees the introduction of the composite sorber in alreadyconsolidated form. A subsequent thermal treatment causes then themelting thereof and consequent filling of the inner volume of the devicein a nearly uniform manner. Among the sensitive devices that benefit themost of this particular configuration are the photovoltaic cells and theOLED displays.

In a third aspect thereof the invention is inherent to composite sorbersconsisting of polymeric matrices comprising dissolved hygroscopicinorganic salts that could be chosen from one or more of the followinginorganic compounds: alkaline-metal and alkaline earth-metalperhalogenates, alkaline-metal and alkaline earth-metal halides, amongwhich the use of perchlorates results to be preferred.

The invention will be further described with reference to the followingexamples:

EXAMPLE 1

A film of a polymeric composite sorber according to the presentinvention is produced by dissolving a hygroscopic inorganic salt in amonomer. A quantity of 0.4 grams of magnesium perchlorate is used, thepowder's particle size of which is not controlled and may also compriseflakes of a diameter of 2 mm. The inorganic salt is dissolved in 5 gramsof methylmetacrylate (MMA) and 0.05 grams of benzoin methyl ether, thislast one having the function of polymerisation initiator.

The polymerisation is carried out inside a chamber model UVACUBE 100produced by Honle, and obtained by means of irradiation with ultravioletlight produced by a 100 W mercury lamp. In the following example, thesolution is subjected to a “pre-UV curing” treatment for 14 minutes witha radiation dose equal to 4.35 J/cm².

The pre-polymerised solution is spread out by means of a filmograph witha thickness of 50 microns on a steel plate and finally consolidated bymeans of a “post-UV curing” treatment for 30 minutes (the correspondingradiation dose being 9.32 J/cm²). All of the preceding operations arecarried out in a glove box under an inert gas flux in order not tocompromise the sorbing capacity of the hygroscopic inorganic salt.

The photography by optical microscopy is shown in FIG. 1.

EXAMPLE 2 Comparative

A film of a polymeric composite sorber is produced by not operatingunder the conditions of the invention i.e. using a common solvent,dichloromethane (CH₂Cl₂), for the polymer (PMMA) and the inorganic salt(Mg(ClO₄)₂), in which the solvent is not able of dissolving theinorganic salt.

The quantities of polymer and inorganic salt are the same as in example1, also the particle size of the inorganic salt is the same, whereas asregards the quantity of the solvent, a quantity equal to 15 g has beenused.

The photography by optical microscopy is shown in FIG. 2.

EXAMPLE 3 Comparative

A film of a polymeric composite sorber is produced by operating as inexample 1, i.e. using a solution given by the monomer (MMA) mixed withpowders of hygroscopic material, but using a hygroscopic material notforeseen by the present invention, i.e. calcium oxide in powder form,added in a quantity equal to 0.4 g, that does not dissolve inmethylmethacrylate.

The photography by optical microscopy is shown in FIG. 3.

EXAMPLE 4

This example compares a secondary characteristic, the transparency, ofthe films from polymeric composite sorbers obtainable with some specialpolymers (among which PMMA) prepared as described in the examples 1-3.

For this characterisation a double beam spectrophotometer Jasco V 570has been used, with a single monochromator, a Hamamatsu detector, usingdeuterium and halogen lamp, scanning velocity 1000 nm/min, passband 1 nmand 0.3 nm accuracy. The characterised area of each polymeric filmcorresponds to the area of the incident light beam on the sample, i.e. arectangle of 1×10 mm².

The results related to the spectral interval 400-700 nm are shown in theFIGS. 4 and 4A in which:

-   -   curve 1 shows the transmission curve of a PMMA film without any        hygroscopic inorganic salt;    -   the curves 2 and 2′ show the transmission curves of a film        prepared according to what described in example 1, respectively        at time 0 and after 23 hours of exposure to air. In order to        distinguish the two curves, which result to be almost perfectly        superimposed, circles have been use to show the points for the        test at time 0, whereas crosses have been used for the test        after 23 hours; and    -   curve 3 shows the transmission curve of a film prepared        according to what described in example 2, i.e. a PMMA film        comprising not perfectly dissolved magnesium perchlorate salts.

In FIG. 4A the comparison between the transparency curve at time 0(curve 2 a) and after 23 hours of exposure to air, (curve 2 a′), isshown, using a very expanded scale for the axis of ordinates in order toappreciate the differences between these curves.

It was not possible to add a curve relating to the transparency of asample prepared according to example 3, because this already at firstsight resulted opaque.

FIGS. 1-3 highlight how only in the case of FIG. 1 there are noparticles or their aggregates in the polymeric composite sorber film, asinstead happens in the case of FIG. 2, in which there is not perfectlydissolved magnesium perchlorate in the film which gives rise to a filmwith irregular characteristics, whereas FIG. 3 shows the calcium oxideparticles enclosed in the polymeric film.

FIG. 4 allows to observe how the morphological differences in thepolymeric composite sorber film can transform also in phenomenologicaldifferences, referring particularly to a secondary characteristic of thehygroscopic film, i.e. its transparency. This secondary characteristicis very useful in the case the sensitive device is an OLED display orplaced in a solar cell.

In fact it can be observed how a hygroscopic film produced according tothe present invention shows a transparency (lines 2, 2′) that is totallycomparable to that of a polymer without inorganic salt (line 1). Furtherthe characteristic of transparency of the hygroscopic film is notcompromised by the H₂O sorbing of the active component containedtherein, i.e. the hygroscopic inorganic salt, and results to be alwaysgreater than 95% in the spectral interval considered.

FIG. 4A shows how there are no significant differences in thetransmission curve of a composite sorber obtained according to themethod of the invention at time zero (curve 2 a) and after 23 ours ofexposition to air (curve 2 a′).

Instead a film containing a hygroscopic inorganic salt not perfectlydissolved in the polymeric matrix, apart from showing in generaldefinitely inferior transmission characteristics (curve 3) with respectto the films of the present invention, has also a high variability ofits transparency characteristics according to the considered sample zoneof the film.

The situation is still more critical if an active component is chosen tobe added to the polymeric matrix that is not among those described inthe present application, as in the case of calcium oxide, that resultsto give rise to an opaque hygroscopic film and for which it was not evenpossible to perform the characterisation.

1. A method for manufacturing a composite sorber for removal of H₂Ocomprising providing a polymeric matrix and a hygroscopic inorganicsalt, and dissolving said hygroscopic inorganic salt inside saidpolymeric matrix.
 2. The method according to claim 1, in which saidhygroscopic inorganic salt is chosen from alkaline-metal perhalogenates,alkaline earth-metal perhalogenates, alkaline-metal halides, andalkaline earth-metal halides.
 3. The method according to claim 2, inwhich said alkaline-metal perhalogenates and alkaline earth-metalperhalogenates are perchlorates.
 4. The method according to claim 1,wherein providing said polymeric matrix comprises forming said polymericmatrix starting from two different polymers, two precursors or a mixtureof a polymer and a precursor.
 5. The method according to claim 4, inwhich one of said polymers or said polymer precursors acts ascross-linking agent for the polymeric matrix.
 6. The method according toclaim 1, further comprising evaporating solvents from a solutioncomprising a polymer of the polymeric matrix and the dissolvedhygroscopic inorganic salt.
 7. The method according to claim 6, in whichsaid solution comprises a common solvent for the polymer and thehygroscopic inorganic salt.
 8. The method according to claim 1, whereindissolving said hygroscopic inorganic salt comprises dissolving saidhygroscopic inorganic salt in a polymer precursor or a mixture of atleast one polymer and one polymer precursor, that constitutes a solventfor the hygroscopic inorganic salt.
 9. The method according to claim 8,in which said polymer precursor is its monomer.
 10. A method for removalof H₂O from a device sensitive to its presence, the method comprisingusing composite sorber manufactured according to the method of claim 1.11. The method according to claim 10, in which said device is an OLEDdisplay.
 12. The method according to claim 10, in which said device is amicroelectromechanical device.
 13. The method according to claim 10, inwhich said device is a solar cell.
 14. The method according to claim 10,in which said device is a lithium battery.
 15. The method according toclaim 10, in which said composite sorber is used in form of a thin filmwith a thickness between 10 and 200 μm.
 16. The method according toclaim 10, in which said composite sorber is dispensed inside said devicebefore consolidating said sorber.
 17. The method according to claim 16,in which dispensation of said composite sorber occurs by serigraphy. 18.The method according to claim 16, in which consolidation of said sorberis a thermal or UV-promoted polymerization.
 19. The method according toclaim 10, in which said polymeric composite sorber is in form ofnanofibres.
 20. The method according to claim 19, in which saidnanofibres are manufactured by electrospinning.
 21. The method accordingto claim 10, in which said sorber is introduced in said sensitive deviceby melting of said composite sorber.
 22. A polymeric composite sorberconsisting of polymeric matrices comprising dissolved hygroscopic saltsof one or more of the following inorganic salts: alkali-metalperhalogenates, alkaline earth-metal perhalogenates, alkali-metalhydroxides, alkali-metal halides and alkaline earth-metal halides. 23.The polymeric composite sorber according to claim 21, in which saidalkali-metal perhalogenates are perchlorates.
 24. The polymericcomposite sorber according to claim 21, in which said polymeric matrixis obtained by combination of two different types of polymers or oftheir precursors or a mixture of at least one polymer and one polymerprecursor.
 25. The polymeric composite sorber according to claim 23, inwhich said one of said polymers or polymer precursors acts ascross-linking agent for said polymeric matrix.